The present invention relates to a control apparatus for an automatic transmission of a motor vehicle in which the optimum running gear, and hence the correct control oil pressure in the transmission, are determined in accordance with signals representing the engine load and vehicle speed.
In such a control apparatus, the engine load and vehicle speed are represented by electric signals. The optimum running gear and corresponding optimum control oil pressure for various running conditions are determined in response to these signals by an electronic arithmetic circuit. The electronic arithmetic circuit drives solenoid valves which control an oil pressure clutch or oil brake of the transmission mechanism so as to smoothly effect gear changes in accordance with the input signals representing the engine load and vehicle speed.
To provide the signals representing the engine load and vehicle speed, typically, devices such as a potentiometer, which produces an electric signal in proportion to the amount of opening of the engine throttle, and a magnetic pickup, which produces a voltage or frequency signal indicative of the rate of rotation of the output shaft of the transmission, are employed. These signals are processed by the electronic arithmetic circuit so as to effect shifting towards lower gears as the amount of the throttle opening increases.
However, if a fault occurs in the speed sensor such that, for instance, its output suddenly falls to zero when the vehicle is operated at a high speed, the transmission will suddenly be shifted to first gear. This is dangerous in that, not only does the engine overrev, but skidding may occur. To overcome this drawback, it has been the practice to detect such a fault and to lock the transmission in an intermediate gear is which the stability is relatively high. However, in the conventional system, this safety measure was effected even if the fault in the output of the speed sensor was temporary, such as may be due to vibration, shock, or electrical surges. Accordingly, the safety circuit tended to be actuated in situations in which such was not really necessary.